In a conventional high dynamic range measurement system of the type used, for example, in shock wave and vibration measurement, the input range setting is one of the most important settings. For example, in an analysis system there may be a number of different input voltage range settings for each input channel. The input range setting has a direct impact on the quality of measurement, which is mainly reflected by SNR (Signal-to-Noise Ratio) or dynamic range. Users are often troubled by being unable to set the optimum range because the measured signal either is non-stationary or has an unknown amplitude. For a high channel count system having multiple input ranges, it is even more difficult to get all the input ranges to a suitable value. To deal with this situation, many instruments are designed with an intelligent auto-ranging capability. “Auto-ranging” tries to set the best input range based on an estimated measurement before the test actually begins. Auto-ranging can only deal with stationary or repetitive signals, i.e., those signals without many magnitude changes. For non-stationary signals such as electrical transients, shock waves, impacts, earthquake signals, and the like, auto-ranging usually does not work because each pulse may take a different magnitude. For a signal with long time history and a large range of amplitude change, auto-ranging cannot be applied at all because during the measurement procedure the signal input range, i.e., the amplifier gain setting, cannot be changed.
As described in the publication “New Technology Increases the Dynamic Ranges of Data Acquisition Systems Based on 24-Bit Technology,” in SOUND AND VIBRATION, April 2005, pages 8-11, Andersen et al. state that sound and vibration transducers (e.g., microphones) have outperformed other analysis systems in linearity and dynamic performance. For such a system, the ratio between the highest and lowest signal level the system can handle is defined as its “dynamic range.” The publication states that if the dynamic range is too low, high range signals will typically be clipped and distorted while the low range signals will typically be buried in system noise that originates from the transducer element and the electronics conditioning the transducer. As a solution, the publication describes utilizing a specialized analog input designed to provide a very high dynamic range of analog circuit pre-conditioning the transducer signal before forwarding the signal to a pair of specially designed 24-bit analog-to-digital converters (ADCs) in two paths. Both data streams from the ACDs are forwarded to a digital signal processing environment, where dedicated algorithms in real-time merge the signals.
In U.S. Pat. No. 7,302,354, assigned to the assignee of this invention, J. Zhuge describes dual A/D (analog-to-digital) signal paths and cross-path amplitude calibration to provide accurate and reliable measurements in a data acquisition system.
In the '354 patent, the input signal is directed to two paths, e.g., Path A and Path B. The first path measures the full range (e.g., +/−10 volts), while the second path includes a high-gain amplifier, such as one having a gain factor of 1024. Each path includes an analog-to-digital converter (ADC). Thus, the preferred embodiment includes a measurement channel with a one-to-one correspondence between the number of paths and the number of ADCs, which sample the input signal simultaneously.
After the ADCs of the different paths convert the input signal into the digital domain, the system selects among measurement points. When the input signal is within the amplitude range of high gain Path B, the system selects the values from Path B. On the other hand, when the magnitude of the input signal is outside the amplitude range of Path B, the system selects the values from Path A. Thus, a subset of measurement points is selected from Path B, the default path, and the remaining measurement points are selected from Path A, so that the selected values at the measurement points are stitched into a final data stream. The total dynamic range of the measurement is increased by roughly 60 dB at full range input.
If Path B will be saturated when a signal is greater than a certain amplitude level, the digitized value from the ADC of Path B should not be used in forming the final data stream. Instead, the value at the corresponding measurement point of Path A is used. The selection of measurements occurs on a point-by-point basis.
There are a number of potential concerns with this implementation. One concern is whether the small phase difference between the different paths will cause difficulties. Previously it was known that by using the same clock source to control the sampling rate of each ADC, the phase match between paths can be optimized.
When addressing this concern, the values that are of greatest importance are those at transition measurement points when the final data stream transitions from one path to another path during a “stitching” process. Without proper treatment, there will be discontinuities at the transitions. The '354 patent uses a special cross-path amplitude calibration process. It is not necessary that the cross-path calibration eliminate, or even reduce, the absolute measurement error of measurement paths. Instead, the calibration is designed to match the errors among the different paths, so that the paths will generate the measurement values as close as possible. This will allow the transition of the signal from one region to another to be very smooth during the “stitching” process.
Cross path amplitude calibration solves the issue of how to adjust the amplitude difference coming from two A/D converters. In an ideal environment and with perfect electronic circuits, there is no phase mismatch between two or multiple A/D converters in different paths. Amplitude adjustments in the time domain would be sufficient. In reality, there is always phase error or phase mismatch between the two paths, in either analog circuitry or inside of the A/D converters. A large mismatch in phase will make the “stitching process” of digital signals coming from two A/D paths difficult.
With current commercially available data acquisition circuitry, when the signals of interest in a lower frequency range, say below 10 kHz range, the phase mismatch is usually insignificant. When the signals of interest are in a higher frequency range, such as 20 kHz or above, the phase mismatch may be more significant.
An object of the invention is to achieve cross path phase calibration in a dual path data acquisition system involving multiple data channels with phase matching.